Re: [PATCH v1 1/2] pid: add pidfd_open()

Oleg Nesterov
2019-05-16T14:27:00Z

On 05/16, Christian Brauner wrote:
>
> With the introduction of pidfds through CLONE_PIDFD it is possible to
> created pidfds at process creation time.

Now I am wondering why do we need CLONE_PIDFD, you can just do

pid = fork();
pidfd_open(pid);

> +SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> +{
> + int fd, ret;
> + struct pid *p;
> + struct task_struct *tsk;
> +
> + if (flags)
> + return -EINVAL;
> +
> + if (pid <= 0)
> + return -EINVAL;
> +
> + p = find_get_pid(pid);
> + if (!p)
> + return -ESRCH;
> +
> + ret = 0;
> + rcu_read_lock();
> + /*
> + * If this returns non-NULL the pid was used as a thread-group
> + * leader. Note, we race with exec here: If it changes the
> + * thread-group leader we might return the old leader.
> + */
> + tsk = pid_task(p, PIDTYPE_TGID);
> + if (!tsk)
> + ret = -ESRCH;
> + rcu_read_unlock();
> +
> + fd = ret ?: pidfd_create(p);
> + put_pid(p);
> + return fd;
> +}

Looks correct, feel free to add Reviewed-by: Oleg Nesterov <oleg@redhat.com>

But why do we need task_struct *tsk?

rcu_read_lock();
if (!pid_task(PIDTYPE_TGID))
ret = -ESRCH;
rcu_read_unlock();

and in fact we do not even need rcu_read_lock(), we could do

// shut up rcu_dereference_check()
rcu_lock_acquire(&rcu_lock_map);
if (!pid_task(PIDTYPE_TGID))
ret = -ESRCH;
rcu_lock_release(&rcu_lock_map);

Well... I won't insist, but the comment about the race with exec looks a bit
confusing to me. It is true, but we do not care at all, we are not going to
use the task_struct returned by pid_task().

Oleg.